Electrochromic material with improved lifetime

ABSTRACT

Disclosed are viologen derivatives as an electrochromic material having improved stability and lifetime, a metal oxide electrode including the same, and an electrochromic device using the viologen derivative as an electrochromic material. The viologen derivative includes a suitable regulator group capable of increasing ΔE that is a potential difference between E 1  (potential at the first redox reaction) and E 2  (potential at the second redox reaction). When ΔE increases, the mole fraction of viologen molecules present in the second reduction state decreases. Therefore, it is possible to lower the mole fraction of viologen molecules present in an irreversibly reduced state at an applied potential, thereby increasing the lifetime of an electrochromic material and an electrochromic device.

TECHNICAL FIELD

The present invention relates to viologen derivatives as anelectrochromic material having improved stability and lifetime, a metaloxide electrode including the same, and an electrochromic device usingthe viologen derivative as an electrochromic material.

BACKGROUND ART

In general, the so-called “electrochromism” is a phenomenon in which acolor change arises depending on potentials of an applied electricfield. Use of the electrochromism results in production ofelectro-photoswitchable devices such as electrochromic devices,information memory devices, and solar cells. Typical electrochromicmaterials include inorganic metal oxides such as tungsten trioxide(WO₃), nickel oxide (NiO) and titanium dioxide (TiO₂), and organicelectrochromic materials including bipyridinium salt (viologen)derivatives, quinone derivatives such as anthraquinone and azinederivatives such as phenothiazine.

Though the electrochromism was known in 1961, practical use andcommercial mass production of electrochromic devices have been limitedbecause of their shortcomings, such shortcomings being that it isdifficult to realize multiple colors, coloring/bleaching rates are low,it is difficult to accomplish complete bleaching, and electrochromicmaterials tend to be damaged easily during repeated coloring/bleachingcycles due to their poor stability.

U.S. Pat. No. 5,441,827 (Graetzel et al.) discloses a device such as aphotocell, photochromic device or an electrochromic device, having highsurface area of an electrode, high concentration of electroactivematerials, high efficiency and fast response speed, the device beingmanufactured by coating an electroactive organic material, as amonolayer, onto the surface of a nanoporous metal oxide thin filmelectrode obtained by sintering metal oxide nanoparticles. The devicesubstantially solves the problems with which electrochromic devicesaccording to the prior art are faced.

PCT International Publication No. WO 98/35267 (Fitzmaurice et al.)discloses an electrochromic device capable of repeatingcoloring/bleaching cycles 10,000 times or more at room temperature, thedevice being a more specified version of the above-mentioned metal oxidethin film-based electrochromic device. However, lifetime ofelectrochromic devices should be increased to 100,000 cycles or more inorder to commercialize electrochromic devices and to expand applicationof electrochromic devices.

DISCLOSURE OF THE INVENTION

Viologen compounds are those containing 4,4′-bipyridinium, and havethree oxidation states, i.e., bipm²⁺, bipm⁺ and bipm⁰, as represented bythe following scheme:

Particularly, the redox reaction of bipm²⁺ bipm⁺ occurs at redoxpotential E₁ and is reversible. Though the redox reaction of bipm⁺⇄bipm⁰occurs at redox potential of E₂, it is frequently irreversible. Bipm⁰ ischemically unstable and thus tends to react with molecular oxygen orother solvent molecules to be transformed into a molecule having adifferent chemical structure, thereby losing its function as anelectrochromic material. Redox reactions of a bipyridinium ion occur atE₁ and E₂, in turn. However, the half wave potentials E₁ and E₂ aremerely the voltage values where oxidation rate becomes equal toreduction rate so as to accomplish a dynamic equilibrium state.Moreover, redox reactions occur not only at the half wave potentials butcontinuously occur at any other potentials, and each of bipm²⁺, bipm⁺and bipm⁰ is present at different mole fractions. Distribution of eachchemical species according to potential follows the BoltzmanDistribution. Therefore, the present inventors have made an attempt todecrease the mole fraction of bipm⁰ at the half wave potential E₁ byincreasing ΔE that is a potential difference between E₁ and E₂. Wethought that a decrease in the mole fraction of bipm⁰ species at thehalf wave potential E₁ might result in improvement in lifetime ofelectrochromic devices, because the state of bipm⁰ is chemicallyunstable and has a strong tendency toward irreversible redox reactions.

Under these circumstances, we introduced various regulator groups intoviologen derivatives to increase ΔE. As a result, we found anelectrochromic material based on a viologen derivative and anelectrochromic device showing higher optical density and having animproved lifetime.

According to an aspect of the present invention, there is provided anelectrochromic material including a viologen compound having a regulatorgroup linked to 4,4′-bipyridinium having three types of oxidationstates, i.e., bipm²⁺, bipm⁺ and bipm⁰, as represented by the followingscheme, the regulator group being capable of increasing ΔE that is apotential difference between E₁ and E₂:

wherein each of E₁ and E₂ represents a redox potential.

According to another aspect of the present invention, there is providedan electrochromic material including a viologen compound having anelectropositively charged cationic regulator group linked to4,4′-bipyridinium.

According to still another aspect of the present invention, there isprovided an electrochromic material including a viologen compound havinga redox couple regulator group linked to 4,4′-bipyridinium, the redoxcouple regulator group being capable of forming a redox coupleelectrically with a bipyridinium ring.

According to still another aspect of the present invention, there areprovided a metal oxide electrode coated with the above-describedelectrochromic material and an electrochromic device including the sameelectrochromic material.

According to still another aspect of the present invention, the presentinvention provides a compound represented by the following formula 1:

wherein each of R¹, R², R⁴ and R⁵ independently or simultaneouslyrepresents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷,CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably, each of R¹, R², R⁴ and R⁵represents H); each of R⁹⁷ and R⁹⁸ represents a C1-C6 alkyl group,preferably a C1-C2 alkyl group; and either or both of linker 1 andlinker 2 may be present, as necessary.

Typical examples of the compound represented by formula 1 include acompound represented by the following formula 1-1:

wherein each of R¹, R², R⁴ and R⁵ independently or simultaneouslyrepresents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷,CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably, each of R¹, R², R⁴ and R⁵represents H); and each of R⁹⁷ and R⁹⁸ represents a C1-C6 alkyl group,preferably a C1-C2 alkyl group.

Hereinafter, the present invention will be explained in more detail.

A viologen compound has a structure in which two pyridinium rings areattached to each other as depicted in the following scheme:

When a viologen compound is present in the state of bipm²⁺, twopyridinium rings are orthogonal to each other and has no resonancestructure between them. Therefore, the state of bipm²⁺ is a very stable,colorless and transparent state. However, while bipm²⁺ accepts anelectron to be reduced to the state of bipm⁻, two pyridinium ringsrotate to be present on the same plane and electric charges aredelocalized according to the occurrence of the resonance between twopyridinium rings, thereby producing a deep color. In the state of bipm⁰,two pyridinium rings form a complete planar structure and sterichindrance is generated between 3,3′-hydrogen atoms, thereby making themolecule unstable. In this state, other molecules such as solventmolecules may cause addition-elimination reactions on carbon atoms ofthe ring. Therefore, the viologen compound may be transformed intocompletely different types of molecules or may be subjected to ringopening in the presence of heat or light to be decomposed intocompletely different molecules, such transformations being irreversible.The resultant compound does not have electrochromic activity any longer.Additionally, when the viologen molecule has a planar structure, anaromatic-aromatic stacking phenomenon may occur due to pi-pi (π-π)interactions. Therefore, adjacent viologen molecules aggregate amongthemselves. Each of the states of bipm⁺ and bipm⁰ having a planarstructure is a high-energy state by nature. Aggregation of high-energymolecules may cause a self-quenching phenomenon and side reactionsincluding polymerization reactions, followed by an irreversibletransformation of an electrochromic material resulting in reducedlifetime in an electrochromic device.

Because such collapses and irreversible changes in viologen derivativesoccur largely in the state of bipm⁰, it is necessary to minimize themole fraction occupied by bipm⁰ at a drive voltage in order to obtain anelectrochromic viologen derivative having longer lifetime.

To accomplish this, according to the technical gist of the presentinvention, a bipyridinium ion is provided with a regulator group at itsend, the regulator group being suitable to stabilize the state of bipm⁺and to prevent bipm⁺ from being transformed into bipm⁰. Such regulatorgroups can increase ΔE that is a potential difference between E₁(electrochemical potential where a transformation into bipm⁺ occurs) andE₂ (electrochemical potential where a transformation into bipm⁰ occurs).

The regulator group preferably increases ΔE by 0.04V or more.

Relative mole fractions of various chemical species follow the BoltzmanDistribution, wherein the number of each chemical species is in directproportion to the electric current used for redox reactions. Moreparticularly, the electric current used for redox reactions at eachelectric potential is determined by the following formula (see, Allen J.Bard, and Larry R. Faulkner, “Electrochemical Methods: Fundamentals andApplications”, John Wiley & Sons, 1980, Chap. 6):I=nFAC ₀*(πD _(o)σ)^(1/2)χ(σt)wherein

-   -   I is the maximum current resulting from redox reactions at a        given applied potential;    -   n is the number of electrons coming in and out according to        redox reactions;    -   F is the Faraday constant;    -   C₀ is the concentration of an oxidative/reductive species in        solution;    -   D₀ is a diffusion coefficient; and    -   χ(σt) is an electric current function resulting from reversible        charge transfer.

Particularly, (π)^(1/2)χ(σt) is a function having an exponentialrelationship with an electric potential difference between appliedpotential and half wave potential. More particularly, whenever theapplied potential varies from the E_(1/2) (half-wave potential) value by20 mV, the function, (π)^(1/2)χ(σt) decreases in the ratio of about 1/2.In other words, whenever ΔE increases by 20 mV (0.02V), the molefraction of bipm⁰ at the applied voltage decreases in the ratio of 1/2.Therefore, when ΔE varies by 40 mV or more, the mole fraction of bipm⁰decreases by ¼ or less of the initial value, and thus it is possible toobserve a significant increase in the lifetime of an electrochemicaldevice.

According to the present invention, the regulator group capable ofincreasing ΔE includes: (1) an cationic functional group; and (2) anadditional redox-coupled functional group capable of forming a redoxcouple electrically with a bipyridinium ring.

An electric potential where redox reactions occur is determined by theenergy level of each oxidation state under a given electric field.

A cationic regulator group increases positive charge density in thewhole molecule and changes the charge density of a bipyridinium ring ateach oxidation state, thereby changing energy level at each oxidationstate. Because a change in energy level at each oxidation state isfollowed by a change in redox potentials, it is possible to control ΔE.Additionally, cationic properties increased by a cationic regulatorgroup can decrease aggregation of adjacent viologen molecules due to theeffect of repulsive force between molecules having the same charge.Furthermore, such increased cationic properties can inhibit aself-quenching phenomenon and side reactions including polymerizationreactions. As a result, it is possible to increase the lifetime of aviologen derivative.

Meanwhile, when the regulator group is an additional redox-coupledfunctional group capable of forming a redox couple electrically with abipyridinium ring, a change in redox states of the regulator groupaffects the charge distribution of the whole molecule and changes theredox potential of a bipyridinium ring. Therefore, it is possible toincrease the lifetime of an electrochromic viologen derivative.

Such regulator groups may be linked directly to a viologen derivativewithout any linker or may be attached to a viologen derivative by meansof a linker (linker 1).

Additionally, the viologen derivative according to the present inventionmay further comprise an anchor group capable of anchoring to a metaloxide electrode so as to show its function sufficiently when it iscoated on the metal oxide electrode for an electrochromic device. Suchanchor groups may be linked directly to a viologen derivative withoutany linker or may be attached to a viologen derivative by means of alinker (linker 2), in the same manner as the regulator group.

Therefore, according to a preferred embodiment of the present invention,there is provided a viologen derivative as an electrochromic material,the viologen derivative being [regulator group]-[linker1]-[bipyridinium(bipm)]-[linker 2]-[anchor group], as depicted in thefollowing formula:

The cationic regulator groups that may be used in the present inventioninclude substituted pyridinium derivatives represented by the followingformula 2, substituted quinolinium derivatives represented by thefollowing formula 3, substituted imidazolium derivatives represented bythe following formula 4 and tetraalkylammonium derivatives representedby the following formula 5:

wherein each of R¹, R², R³, R⁴ and R⁵ independently or simultaneouslyrepresents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷,CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably, each of R¹, R², R⁴ and R⁵represents H, and R³ represents N(CH₃)₂ or OR⁹⁷); and each of R⁹⁷ andR⁹⁸ represents a C1-C6 alkyl group, preferably a C1-C2 alkyl group.

wherein each of R⁶, R⁷, R⁸, R⁹, R¹⁰, R¹¹ and R¹² independently orsimultaneously represents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂,COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably each of R⁶, R⁷, R⁹,R¹⁰, R¹¹ and R¹² represents H, and R⁸ represents N(CH₃)₂ or OR⁹⁷); andeach of R⁹⁷ and R⁹⁸ represents a C1-C6 alkyl group, preferably a C1-C2alkyl group.

wherein each of R¹³, R¹⁴, R¹⁵ and R¹⁶ independently or simultaneouslyrepresents H or a C1-C6 alkyl group (preferably, each of R¹³, R¹⁵ andR¹⁶ represents H and R¹⁴ represents a C1-C6 alkyl group).

wherein each of R¹⁷, R¹⁸ and R¹⁹ independently or simultaneouslyrepresents H or a C1-C12 alkyl group, preferably a C1-C4 alkyl group.

The redox couple-functional groups that may be used in the presentinvention include ferrocene derivatives represented by the followingformula 6; azine derivatives represented by the following formulae 7 and8, including phenothiazines, phenoxazines and phenazines; quinonederivatives represented by the following formulae 9-13, includingbenzoquinones, hydroquinones, naphtoquinones, anthraquinones andacenaphthene quinones (formula 13); and multicyclic aromatic compoundsincluding pyrenes represented by the following formula 14, perylenesrepresented by the following formula 15 and dancyls represented by thefollowing formula 16:

wherein each of R²⁰ to R²⁸ independently or simultaneously represents Hor a C1-C6 alkyl group (preferably, all of R²⁰ to R²⁸ simultaneouslyrepresents H or methyl) ; X represents CH₂, O, S, NH, NR⁹⁸ or CO₂; andR⁹⁸ represents a C1-C6 alkyl group, preferably a C1-C2 alkyl group.

wherein X represents S, O or Se; each of R²⁹ to R³⁶ independently orsimultaneously represents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂,COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably, each of R²⁹, R³⁰,R³², R³³, R³⁵ and R³⁶ represents H and each of R³¹ and R³⁴ representsBr, NR⁹⁸ ₂ or OR⁹⁷); and each of R⁹⁷ and R⁹⁸ represents a C1-C6 alkylgroup, preferably a C1-C2 alkyl group.

wherein each of R³⁷ to R⁴⁵ independently or simultaneously represents H,C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ orNR⁹⁸ ₂ (preferably, each of R³⁷, R⁴⁰, R⁴¹ and R⁴⁴ represents H, each ofR³⁸, R³⁹, R⁴² and R⁴³ represents Br, NR⁹⁸ ₂ or OR⁹⁷, and R⁴⁵ representsa C1-C6 alkyl group); and each of R⁹⁷ and R⁹⁸ represents a C1-C6 alkylgroup, preferably a C1-C2 alkyl group.

wherein each of R⁴⁶ to R⁶³ independently or simultaneously represents H,a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ orNR⁹⁸ ₂ (preferably, all of R⁴⁶ to R⁶³ simultaneously represents H); Xrepresents CH₂, O, S, NH, NR⁹⁸ or CO₂ (wherein the position of X may beeither of a-position and b-position of anthracene in the case ofanthraquinone); and each of R⁹⁷ and R⁹⁸ represents a C1-C6 alkyl group,preferably a C1-C2 alkyl group.

wherein X represents CH₂, O, S, NH, NR⁹⁸ or CO₂; each of R⁸⁴ to R⁸⁸independently or simultaneously represents H, a C1-C6 alkyl group, OH,OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably, allof R⁸⁴ to R⁸⁸ simultaneously represents H); and each of R⁹⁷ and R⁹⁸represents a C1-C6 alkyl group, preferably a C1-C2 alkyl group.

wherein X represents CH₂, O, S, NH, NR⁹⁸ or CO₂; each of R⁶⁴ to R⁷²independently or simultaneously represents H, a C1-C6 alkyl group, OH,OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably, allof R⁶⁴ to R⁷² simultaneously represents H) ; and each of R⁹⁷ and R⁹⁸represents a C1-C6 alkyl group, preferably a C1-C2 alkyl group.

wherein X represents CH₂, O, S, NH, NR⁹⁸ or CO₂; each of R⁷³ to R⁸³independently or simultaneously represents H, a C1-C6 alkyl group, OH,OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂ (preferably, allof R⁷³ to R⁸³ simultaneously represents H); and each of R⁹⁷ and R⁹⁸represents a C1-C6 alkyl group, preferably a C1-C2 alkyl group.

wherein each of R⁸⁹ to R⁹⁴ independently or simultaneously represents H,a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ orNR⁹⁸ ₂ (preferably, all of R⁸⁹ to R⁹⁴ simultaneously represents H); eachof R⁹⁵ and R⁹⁶ independently or simultaneously represents H or a C1-C6alkyl group, preferably a C1-C2 alkyl group; and each of R⁹⁷ and R⁹⁸represents a C1-C6 alkyl group, preferably a C1-C2 alkyl group.

Anchor groups that may be used in the present invention includephosphonic acid represented by the following formula 17, salicylic acidrepresented by the following formula 18, boronic acid represented by thefollowing formula 19, iminodiacetic acid represented by the followingformula 20 and ortho-dihydroxyaryl (catechol) represented by thefollowing formula 21:

wherein X may be O, NH, NR⁹⁸, S or CO, and R⁹⁸ represents a C1-C6 alkylgroup, preferably a C1-C2 alkyl group.

As described above, regulator groups or anchor groups may be linkeddirectly to a bipyridinium ring without any linker or linked to abipyridinium ring by means of a linker. When a linker is used, thelinker (linker 1, linker 2) may be an C1-C4 alkyl chain represented bythe following formula 22, xylene (—CH₂—Ar—CH₂—) represented by thefollowing formula 23, 1,3,5-triazine (C₃N₃) represented by the followingformula 24, or a substituted aromatic ring represented by the followingformula 25. When the linker is an aromatic ring linker, the link may bepresent at ortho-, meta- and para-positions.

wherein n is an integer of between 1 and 4.

wherein X may be O, NH, NR⁹⁸, S or CO, and R⁹⁸ represents a C1-C6 alkylgroup, preferably a C1-C2 alkyl group.

Counterions of the viologen derivative according to the presentinvention may include Cl⁻, Br⁻, BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻ and (CF₃SO₂)₂N⁻.

In general, electrochromic materials according to the present inventionmay be prepared as follows: 4,4′-bipyridine is reacted with oneequivalent of an anchor group to obtain a unit formed of bipyridinehaving an anchor group at one end thereof, i.e., a unit ofbipyridinium-(linker)-anchor group. The anchor group may be protectedwith an ester or ketal protecting group, etc. To accomplish the link,various types of reactions including a nucleophilic substitutionreaction, esterification reaction, addition-elimination reaction andmetal catalytic reaction may be used depending on the kind of linker.When an addition-elimination reaction or metal catalytic reaction isused, the link can be made without any linker. Next, a regulator groupis linked to the pyridine ring remaining in the resultant unit ofbipyridinium-(linker)-anchor group, directly without any linker or byusing a linker, to form a molecule of regulatorgroup-bipyridinium-anchor group. Then, deprotection of the anchor groupmay be performed to activate the molecule, if necessary. By doing so, itis possible to obtain an electrochromic material having a regulatorgroup, which is capable of being bonded to an electrode.

The electrochromic device according to the present invention includes afirst electrode disposed on a transparent or translucent substrate, asecond electrode and an electrolyte, wherein at least one of the firstelectrode, second electrode and electrolyte includes the electrochromicmaterial according to the present invention.

The electrodes and the electrochromic device may be manufactured byconventional methods known to one skilled in the art, except that theelectrochromic material according to the present invention is used (see,U.S. Pat. No. 5,441,827 and PCT International Publication No. WO98/35267).

Hereinafter, a preferred embodiment of the method for manufacturing theelectrochromic device according to the present invention will bedescribed.

A nanoporous metal oxide electrode that may be used in the presentinvention is prepared as follows: Nanocrystalline metal oxide particleshaving an average particle size of 2-200 nm were suspended in an organicsolvent along with an organic binder to form a paste. Metal oxides thatmay be used include oxides of a metal selected from the group consistingof titanium, zirconium, hafnium, chrome, molybdenum, tungsten, vanadium,niobium, tantalum, silver, zinc, strontium, iron (Fe²⁺ and Fe³⁺), nickeland perovskites thereof. Preferably, the metal oxide is TiO₂, WO₃, MoO₃,ZnO, SnO₂, indium-doped tin oxide or indium-doped zinc oxide. Theorganic binder has a molecular weight of between several thousands andseveral millions. Particular examples of the organic binder includealkyl cellulose, dextran, PMMA (poly(methyl methacrylate)) and Carbowax.The organic solvents that may be used include methanol, ethanol,isopropyl alcohol, dimethylglycol dimetylether, propyleneglycolpropylether, propylene glycolmethylether acetate and terpineol. Thepaste is printed on the surface of a conductive electrode by using aprinting method such as screen printing, stencil printing, spin coatingor doctor blading. The conductive electrode may be an ITO or FTO thinfilm electrode coated on the surface of glass, or a metal electrode suchas gold, silver, aluminum, copper, chrome, chrome/silver alloy orsilver/palladium alloy. Then, the resultant assembly of [metal oxidenanoparticle-organic binder/conductive electrode] is sintered at hightemperature so as to burn out the organic binder and thus to formnanopores, while the metal oxide nanoparticles are interconnected toform a porous metal oxide electrode. Then, the electrochromic materialaccording to the present invention is coated on the resultant system of[nanoporous metal oxide electrode/conductive electrode] by using aself-assembly process, thereby providing a working electrode for anelectrochromic device.

Counter electrodes that may be used include a nanoporous metal oxideelectrode obtained as described above, an ITO or FTO thin film electrodecoated on the surface of glass, or a metal electrode such as gold,silver, aluminum, copper, chrome, chrome/silver alloy orsilver/palladium alloy. A white reflective plate may be optionallyinserted between the working electrode and the counter electrode. Thewhite reflective plate may be formed by coating titania or silicananoparticles having a size of between 200 nm and 600 nm on the surfaceof counter electrode and then sintering the coated electrode at atemperature of 200° C. or higher.

The electrochromic device according to the present invention may bemanufactured by laminating the working electrode with the counterelectrode, obtained as described above, by means of an adhesive,injecting an electrolyte and sealing the device. Electrolytes that maybe used include liquid electrolytes containing a lithium salt ortetraalkylammonium salt dissolved in a solvent, ionic liquids, gelledlithium salt electrolytes, gelled ionic liquids and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing variations in redox potentials of the viologenderivatives obtained from Example 1 and Comparative Examples 1 and 2.

FIG. 2 is a schematic view showing the structure of an electrochromicdevice including a metal oxide electrode coated with the viologenderivative obtained from Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only and the present invention is not limited thereto.

EXAMPLE 12-((α′(4′″-N,N-dimethylpyridinium)-4″-α-)-4,4′-bipyridinium)-ethylphosphonic acid trichloride salt (III) (PV-DMAP)

<Synthesis of PV-DMAP>

1-(4′-Bromomethyl-benzyl)-4-dimethylamino-pyridinium bromide (I):

30 ml of THF solution containing 1 g of 4-dimethylaminopyridinedissolved therein was added gradually to 100 ml of THF solutioncontaining 4.32 g of dibromo-p-xylene dissolved therein at 4° C. and themixture was reacted for 2 hours to form precipitate. After filtration,the precipitate was dried under vacuum to obtain 3.23 g of compound (I).

¹H-NMR (DMSO-d₆; ppm): 8.43(2H), 7.48(2H), 7.39(2H), 7.06(2H), 5.42(2H),4.70(2H), 3.18(6H) ; MS(LC) : m/z=305 (M⁺).

N-(phosphono-2-ethyl)-4″-dimethylamino-pyridinium-4,4′-bipyridiniumtribromide (II):

4.60 g of compound (I) prepared as described above and 4.00 g ofN-(diethylphosphono-2-ethyl)-4,4′-bipyridinium bromide were dissolved in100 ml of CH₃CN and the reaction mixture was reacted under reflux for 24hours. After filtration, the precipitate was dried under vacuum toobtain 6.81 g of compound (II).

¹H-NMR (D₂O; ppm): 9.22 (2H), 9.19(2H), 8.64(2H), 8.60(2H), 8.09(2H,d),7.60(2H,d), 7.48(2H), 6.93(2H), 6.00(2H), 5.42(2H), 5.07(2H), 4.17(4H),3.23(6H), 2.86(2H), 1.29(6H).

N-(phosphono-2-ethyl)-4″-dimethylamino-pyridinium-4,4′-bipyridiniumtrichloride (III):

6.81 g of compound (II) prepared as described above was dissolved in 50ml of 6N HCl and reacted under reflux for 24 hours. After evaporation ofthe solvent, recrystallization was performed by using H₂O, MeOH and THFto obtain 5.98 g of compound (III).

¹H-NMR (D₂O; ppm): 9.23 (4H), 8.63(4H), 8.12(2H), 7.65(2H), 7.53(2H),6.03(2H), 5.45(2H), 5.01(2H), 3.26(6H), 2.57(2H) MS(LC): m/z=489 (M⁺).

EXPERIMENTAL EXAMPLE

<Measurement of Redox Potentials of Viologen Derivative>

The redox potentials of compound (III) in solution were measured bycyclic voltametry. Particularly, cyclic current-voltage curve wasdetermined in aqueous 0.5M KCl solution by using a glassy carbonelectrode as a working electrode, Pt electrode as a counter electrodeand Ag/AgCl as a reference electrode. As shown in the following Table 1and FIG. 1, it was possible to observe the first stage of reduction at−0.520 V (E₁, reversible) and the second stage of reduction at −0.975 V(E₂, irreversible).

<Manufacture of Electrochromic Device Including Metal Oxide ElectrodeCoated with Viologen Derivative and Determination of Lifetime>

Ti(O-iPr)₄ was hydrolyzed to form a colloidal dispersion of TiO₂nanoparticles. The nanoparticles that were initially formed had anaverage size of 7 nm. The nanoparticles were autoclaved at 200° C. for12 hours to increase the average size to 12 mm. The solvent wasdistilled under reduced pressure to the concentration of 160 g/l. Then40 wt % of Carbowax 20000 (poly(ethylene oxide) having an averagemolecular weight of 20,000) based on the weight of TiO₂ was added to thesolution, thereby forming white titania sol slurry with high viscosity.The slurry was printed on an ITO transparent electrode by using a screenprinting process and the printed electrode was sintered at a hightemperature of 450° C. to provide a transparent electrode based on TiO₂having nanopores as a working electrode. As a counter electrode, anantimony-doped tin oxide (Sb-doped SnO₂) electrode was formed in asimilar manner. The surface of the counter electrode was further coatedwith TiO₂ nanoparticles present in the rutile phase by using a screenprinting process and then sintered to form a reflective plate.

The transparent working electrode obtained as described above wasimmersed in 50 ml of 10 mM aqueous solution of compound (III) for 30minutes and then washed with 50 ml of ethanol two times. The workingelectrode was dried at room temperature for 4 hours, and then athermosetting adhesive was applied on the working electrode so as to beintegrated with the counter electrode. Gamma-butyrolactone solutioncontaining 10 mM of LiClO₄ was injected as an electrolyte and theresultant device was sealed by UV curing. The resultant electrochromicdevice (FIG. 2) developed deep blue purple color at 1.0V and showed nodeterioration even after 500,000 times of coloring/bleaching cycles.

COMPARATIVE EXAMPLE 1 2-(4-Benzyl-,4′-bipyridinium)ethyl phosphonic aciddichloride salt (VI) (PVB)

3.12 g of 4,4′-Dipyridyl was mixed with 5.10 g of bisethyl-2-bromoethylphosphonate and the mixture was reacted for 12 hours at roomtemperature. 300 ml of cold diethylether was added thereto, followed bystirring for additional 1 hour and filtration of precipitate. Theprecipitate was washed with 50 ml of diethylether three times and driedunder vacuum to obtain 6.21 g of compound (IV).

To 400 ml of CH₃CN containing 6.21 g of compound (IV) dissolved therein,3.70 g of benzyl bromide was added. Next, the mixture was stirred underreflux for 6 hours at 80° C. The reaction mixture was cooled to roomtemperature and poured into 300 ml of cold diethylether, followed bystirring for additional 1 hour and filtration of precipitate. Theprecipitate was washed with 50 ml of CH₃CN three times and dried undervacuum to obtain 7.44 g of compound (V).

¹H-NMR (DMSO-d₆; ppm): 9.58 (2H), 9.45(2H), 8.83(4H), 7.65(2H),7.46(3H), 5.99(2H), 4.91(2H), 4.02(4H), 2.76(2H), 1.21(6H).

7.44 g of compound (V) was dissolved in 150 ml of 6N HCl and reactedunder reflux for 24 hours. After evaporation of the solvent,recrystallization was performed by using H₂O, MeOH and THF to obtain5.27 g of compound (VI).

¹H-NMR (DMSO-d₆; ppm): 9.59 (2H), 9.43(2H), 8.84(2H), 8.80(2H),7.65(2H), 7.46(3H), 5.98(2H), 4.89(2H), 2.63(2H).

As shown in Table 1 and FIG. 1, compound (VI) showed the first reductionat −0.586 V (E₁, reversible) and the second reduction at −0.879 V (E₂,irreversible), under the same conditions as Example 1. An electrochromicdevice was manufactured in the same manner as Example 1, except thatcompound (VI) was used instead of compound (III). The resultantelectrochromic device showed intense bluish purple color at 1.0V andshowed deterioration after 10,000 times of coloring/bleaching cycles.

COMPARATIVE EXAMPLE 2 4,4′-bipyridinium-bis(diethyl phosphonicacid)dichloride salt (VIII) (PVP)

3.12 g of 4,4′-Dipyridyl was mixed with 10.1 g ofdiethyl-(2-bromoethyl)phosphonate and the mixture was reacted for 12hours at room temperature. 500 ml of cold diethylether was addedthereto, followed by stirring for additional 1 hour and filtration ofprecipitate. The precipitate was washed with 50 ml of diethylether threetimes and dried under vacuum to obtain 11.6 g of compound (VII). 11.6 gof compound (VII) obtained as described above was dissolved in 100 ml of6N HCl and reacted under reflux for 24 hours. After evaporation of thesolvent, recrystallization was performed by using H₂O, MeOH and THF toobtain 6.85 g of compound (VIII).

As shown in Table 1 and FIG. 1, compound (VIII) showed the firstreduction at −0.630 V (E₁, reversible) and the second reduction at−0.955 V (E₂, irreversible), when an Ag/AgCl reference electrode wasused. An electrochromic device was manufactured in the same manner asExample 1, except that compound (VIII) was used instead of compound(III). The resultant electrochromic device developed deep blue color at1.3V and showed deterioration after 10,000 times of coloring/bleachingcycles. TABLE 1 Potential Viologen V²⁺ → V¹⁺ → Difference Derivative V¹⁺(E₁) V⁰ (E₂) (ΔE) Ex. 1 PV-DMAP −0.520 −0.975 0.455 Comp. Ex. 1 PVB−0.586 −0.879 0.293 Comp. Ex. 2 PVP −0.630 −0.955 0.325

INDUSTRIAL APPLICATIONS

As can be seen from the foregoing, the viologen derivative according tothe present invention is provided with a suitable regulator groupcapable of increasing ΔE at one end thereof. The viologen derivative canincrease the lifetime of an electrochromic device by 3-10 times at a lowdrive voltage of 1.0V. Additionally, the electrochromic materialaccording to the present invention shows an improved optical densitycompared to conventional electrochromic materials.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings. On the contrary, it is intendedto cover various modifications and variations within the spirit andscope of the appended claims.

1. An electrochromic material, which comprises a viologen compoundhaving a regulator group linked to 4,4′-bipyridinium having threeoxidation states, bipm²⁺, bipm⁺ and bipm^(o), as represented by thefollowing scheme, the regulator group being capable of increasing ΔEthat is a potential difference between E₁ and E₂:

wherein each of E₁ and E₂ is a redox potential.
 2. An electrochromicmaterial, which comprises a group viologen compound having a cationicregulator group linked to 4,4′-bipyridinium.
 3. An electrochromicmaterial, which comprises a viologen compound having an additionalredox-coupled functional regulator group linked to 4,4′-bipyridinium,the regulator group being capable of forming a redox couple electricallywith a bipyridinium ring.
 4. The electrochromic material according toclaim 1, wherein the regulator group is capable of increasing ΔE by0.04V or more.
 5. The electrochromic material according to any one ofclaims 1 to 3, wherein the viologen compound further includes acounterion, the counterion being Cl⁻, Br⁻, BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻ or(CF₃SO₂)₂N⁻.
 6. The electrochromic material according to any one ofclaims 1 to 3, wherein the regulator group is linked directly to4,4′-bipyridinium without any linker, or linked to 4,4′-bipyridinium bymeans of a linker (linker 1).
 7. The electrochromic material accordingto any one of claims 1 to 3, wherein an anchor group capable of beingbonded to a metal oxide electrode is further linked to4,4′-bipyridinium, the anchor group being linked directly to4,4′-bipyridinium without any linker, or linked to 4,4′-bipyridinium bymeans of a linker (linker 2).
 8. The electrochromic material accordingto claim 1 or 2, wherein the regulator group is selected from the groupconsisting of functional groups represented by the following formulae 2to 5:

wherein each of R¹, R², R³, R⁴ and R⁵ independently or simultaneouslyrepresents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷,CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; and each of R⁹⁷ and R⁹⁸ represents a C1-C6alkyl group.

wherein each of R⁶, R⁷, R⁸, R⁹, R¹⁰, R¹¹ and R¹² independently orsimultaneously represents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂,COON, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; and each of R⁹⁷ and R⁹⁸represents a C1-C6 alkyl group.

wherein each of R13, R14, R15 and R16 independently or simultaneouslyrepresents H or a C1-C6 alkyl group.

wherein each of R¹⁷, R¹⁸ and R¹⁹ independently or simultaneouslyrepresents H or a C1-C12 alkyl group.
 9. The electrochromic materialaccording to claim 1 or 3, wherein the regulator group is selected fromthe group consisting of functional groups represented by the followingformulae 6 to 16:

wherein each of R²⁰ to R²⁸ independently or simultaneously represents Hor a C1-C6 alkyl group; X represents CH₂, O, S, NH, NR⁹⁸ or CO₂; and R⁹⁸represents a C1-C6 alkyl group.

wherein X represents S, O or Se; each of R²⁹ to R³⁶ independently orsimultaneously represents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂,COON, C0₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; and each of R⁹⁷ and R⁹⁸represents a C1-C6 alkyl group.

wherein each of R³⁷ to R⁴⁵ independently or simultaneously represents H,a C1-C6 alkyl group, OH, OR97, CN, N02, COON, COZR97, CONHZ, CONR982 orNR982; and each of R⁹⁷ and R⁹⁸ represents a C1-C6 alkyl group.

wherein each of R⁴⁶ to R⁶³ independently or simultaneously represents H,a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ orNR⁹⁸ ₂; X represents CH₂, O, S, NH, NR⁹⁸ or CO₂; and each of R⁹⁷ and R⁹⁸represents a C1-C6 alkyl group.

wherein X represents CH₂, O, S, NH, NR⁹⁸ or CO₂; each of R⁸⁴ to R⁸⁸independently or simultaneously represents H, a C1-C6 alkyl group, OH,OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; and each of R⁹⁷and R⁹⁸ represents a C1-C6 alkyl group.

wherein X represents CH₂, O, S, NH, NR⁹⁸ or CO2; each of R⁶⁴ to R⁷²independently or simultaneously represents H, a C1-C6 alkyl group, OH,OR⁹⁷, CN, NO₂, COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; and each of R⁹⁷and R⁹⁸ represents a C1-C6 alkyl group.

wherein X represents CH2, O, S, NH, NR⁹⁸ or CO₂; each of R⁷³ to R⁸³independently or simultaneously represents H, a C1-C6 alkyl group, OH,OR⁹⁷, CN, NO₂ COOH, CO₂R⁹⁷, CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; and each of R⁹⁷and R⁹⁸ represents a C1-C6 alkyl group.

wherein each of R⁸⁹ to R⁹⁴ independently or simultaneously represents H,a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COON, CO₂R⁹⁷, CONH₂, CON⁹⁸ ₂ orNR⁹⁸ ₂; each of R⁹⁵ and R⁹⁶ independently or simultaneously represents Hor a C1-C6 alkyl group; and each of R97 and R98 represents a C1-C6 alkylgroup.
 10. The electrochromic material according to claim 7, wherein theanchor group is selected from the group consisting of functional groupsrepresented by the following formulae 17 to 21:

wherein X represents O, NH, NR⁹⁸, S or CO, and R⁹⁸ represents a C1-C6alkyl group.
 11. The electrochromic material according to claim 6,wherein the linker is represented by any one formula selected from thegroup consisting of the following formulae 22 to 25:

wherein n is an integer of between 1 and 4 [formula 23]

wherein X represents O, NH, NR⁹⁸, S or CO, and R⁹⁸ represents a C1-C6alkyl group.
 12. A compound represented by the following formula 1:

wherein each of R¹, R², R⁴ and R⁵ independently or simultaneouslyrepresents H, a C1-C6 alkyl group, OH, OR⁹⁷, CN, NO₂, COOH, C0₂R⁹⁷,CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; each of R⁹⁷ and R⁹⁸ represents a C1-C6 alkylgroup; and either or both of linker 1 and linker 2 may be present, asnecessary.
 13. The compound according to claim 12, wherein the compoundis represented by the following formula 1-1:

wherein each of R¹, R², R⁴ and R⁵ independently or simultaneouslyrepresents H, a C1-C6 alkyl group, OH, OR97, CN, NO₂, COOH, C0₂R⁹⁷,CONH₂, CONR⁹⁸ ₂ or NR⁹⁸ ₂; and each of R9′ and R⁹B represents a C1-C6alkyl group.
 14. A metal oxide electrode coated with an electrochromicmaterial as defined in any one of claims 1 to
 3. 15. An electrochromicdevice comprising a first electrode disposed on a transparent ortranslucent substrate, a second electrode and an electrolyte, wherein atleast one of the first electrode, second electrode and electrolytecomprises an electrochromic material as defined in any one of claims 1to
 3. 16. The electrochromic material according to claim 7, wherein thelinker is represented by any one formula selected from the groupconsisting of the following formulae 22 to 25:

wherein n is an integer of between 1 and 4 [formula 23]

wherein X represents O, NH, NR⁹⁸, S or CO, and R⁹⁸ represents a C1-C6alkyl group.